外观模式-Facade

外观模式是为了解决类与类之间依赖关系的，外观模式将类间关系放在一个Facade类中，降低了类类之间的耦合度，该模式中不涉及接口

举一个经典的例子：

CPU类：

 public class CPU {
     public void startup(){
         System.out.println("cpu startup");
     }  

     public void shutdown(){
         System.out.println("cpu shutdown");
     }
 }  

Mainboard类：

 public class Mainboard {  

     public void startup(){
         System.out.println("mainboard startup");
     }  

     public void shutdown(){
         System.out.println("mainboard shutdown");
     }
 } 

Disk类：

 public class Disk {  

     public void startup(){
         System.out.println("disk startup");
     }  

     public void shutdown(){
         System.out.println("disk shutdown");
     }
 } 

Computer类（外观类）：

 public class Computer {
     private CPU cpu;
     private Mainboard mainboard;
     private Disk disk;  

     public Computer(){
         cpu = new CPU();
         mainboard = new Mainboard();
         disk = new Disk();
     }  

     public void startup(){
         System.out.println("starting the computer");
         cpu.startup();
         mainboard.startup();
         disk.startup();
         System.out.println("computer is started");
     }  

     public void shutdown(){
         System.out.println("closing the computer");
         disk.shutdown();
         mainboard.shutdown();
         cpu.shutdown();
         System.out.println("computer is closed");
     }
 } 

测试类：

 public class Main {  

     public static void main(String[] args) {
         Computer computer = new Computer();
         computer.startup();
         computer.shutdown();
     }
 }

测试结果：

starting the computer
cpu startup
mainboard startup
disk startup
computer is started
closing the computer
disk shutdown
mainboard shutdown
cpu shutdown
computer is closed

如果我们没有外观类（Computer），CPU、Mainboard、Disk之间将会相互持有实例对象，造成严重的依赖；而有了Computer，他们之间的关系被放在了Computer类里，这样就起到了解耦的作用。这就是外观模式！